Optical fiber based wireless access schemes have become very popular recently because of their potential to increase system capacity, enable wideband access and to cover special areas such as tunnels and supermarkets. These schemes are especially useful for indoor applications with micro and pico cellular architecture.
When the fiber is short (say, less than a few kilometers) and the radio frequency is only a few GHz, effects of fiber dispersion and laser chirp are negligible [1]. All cited references are listed at the end of this patent disclosure. This is especially true at 1310 nm. Therefore, the ROF link has adequate bandwidth to support wireless multimedia services. In this case, however, nonlinear distortion of the electrical to optical conversion process becomes the major limitation. The impairment is severe in the uplink where, the received signal largely fluctuates due to multipath fading of the wireless channel. Both direct modulation and external modulation schemes suffer from limited dynamic range because of this nonlinear distortion. We focus on AM-AM and AM-PM type nonlinear distortion considering the whole ROF link. However, clipping is neglected.
There have been several attempts to increase the linearity of the ROF link by fixed electronic means [2]. However, fixed schemes suffer from device dependency. We did some work focusing on adaptive compensation of the ROF link nonlinearity, assuming an AWGN wireless channel [3]. However, for a realistic solution wireless channel fading has to be considered also. Recently, we reported an algorithm to identify both the dispersive wireless channel and the nonlinear fiber channel [4].
The nonlinearity of the ROF link contributes to various kinds of impairment. When the negative peak of the modulating signal goes below threshold level, a clipping distortion occurs. In a multicarrier environment, intermodulation distortion also occurs. In addition, even with a single carrier and no clipping, the mild in-band nonlinearity causes AM-AM and AM-PM type nonlinear distortion.
There have been several attempts to increase the linearity of the ROF link by fixed electronic means. These are discussed in detail in [5]. Recently, some work has been done focusing on adaptive compensation of the ROF link nonlinearity, assuming an AWGN wireless channel [3], [6]. However, a good estimation of not only the nonlinearity, but also the multipath wireless channel is essential for efficient equalization.
The fiber-wireless uplink can be modeled as a Wiener system. Reputed mathematician N. Wiener first showed that any BIBO stable nonlinear system with finite memory can be modeled as a Wiener system for Gaussian inputs which, consists of orthogonal linear dynamic functions followed by static nonlinear functions [7].
However, due to the practical difficulties in generating Gaussian inputs, different approaches have been proposed. Pseudorandom (PN) sequences have white noise like properties and, easy to generate and analyze. Their correlation properties are well understood [8]. Besides, maximal length PN sequences are widely used in spread spectrum communications. Therefore, using PN sequence for channel estimation is very attractive in wireless communications. Billings and Fakhouri initially used PN sequences for control system identification [9].